In the field of radio communications, a MIMO (Multiple-Input and Multiple-Output) transmission scheme is being utilized, which realizes improvement in the speed and quality of signal transmission by performing transmission and reception by use of multiple antennas in both the radio transmitting station and the radio receiving station.
In order to further increase the speed and reduce interference in signal transmission, the use of a massive MIMO transmission scheme, which uses a large number of antenna elements (e.g., 100 or more elements), is being considered in a high frequency band (e.g., at 10 GHz or higher) in which reductions in sizes of antennas and securing of wide bandwidths are possible (see, for example, Patent Document 1). For example, the use of the massive MIMO transmission scheme is being considered for the mobile communication system supporting UMTS (Universal Mobile Telecommunications System) LTE-A and subsequently developed systems.
In massive MIMO, advanced precoding using a larger number of antenna elements can be performed, compared with conventional MIMO. In this specification, precoding is a technique of adjusting, by giving weights (weight coefficients) to electrical signals that are to be supplied to the antenna elements, the phases and amplitudes of the electrical signals to control the directions of radio-wave beams emitted from the antenna elements, in order to perform beamforming and to transmit multiple streams that are spatially separated. Beamforming is a technique of controlling the directivity and the shape of a beam by controlling multiple antenna elements. Since the phase and the amplitude can be controlled for each transmitting antenna element in MIMO, the flexibility of beam control improves with the number of antenna elements that are used. The weights for precoding (precoding weights) are selected on the basis of channel state information (CSI) on a transmission path between a radio transmitting station and a radio receiving station.